Titanosilicates are class of solid oxidation catalysts. Crystalline, microporous titanium silicalites (TS-1 and TS-2; U.S. Pat. Nos. 4,410,501 and 1,001,038 and Reddy et al., Appl. Catal. Vol. 58, page L1, year 1990) exhibit highly efficient catalytic activity for oxidation of organic molecules. However, their applicability is limited to only organic molecules with less than 0.5 nm diameter. Mesoporous titanosilicates should overcome this limitation in applicability. However, Ti substituted in mesoporous M41S family silica materials discovered in subsequent years (Corma et al., J. Chem. Soc. Chem. Commun. Page 147, year 1994; Tanev et al., Nature, vol. 368, page 321, year 1994) are thermally less stable (structure collapses at 873 K and in presence of water). While issues related to pore size limitation have been over come with these materials, they exhibit oxidation activity/selectivity lower than those of microporous TS-1 and TS-2. Their thinner pore walls are the cause for their low thermal stability. Unlike in TS-1, Ti in M41S material is located mostly on the pore walls and not substituted in silica framework locations. This is one of the main reasons for their low catalytic activity/selectivity. Moreover, Ti-MCM-41 is only a two-dimensional pore material. Ti—SBA-15 has thicker wall but it is also having a two-dimensional pore architecture (Morey et al., Chem. Mater. Vol. 12, page 898, year 2000). Most of the titanosilicates reported so far have been prepared by post-synthesis methods wherein a structured silica matrix is prepared first and then, titanium is loaded by grafting techniques on the silica structure. In all those materials Ti is located on the pore walls as extra framework nano titanium oxide phase. Such nano-oxide phases are less reactive in selective oxidation reactions. Materials prepared by the in-situ and direct synthesis method can lead to Ti substituted in the framework of silica possessing a tetrahedral geometry. Several mesoporous titanosilicates with different structural and pore architectures have been reported and prepared by direct synthesis methods in basic conditions. They all have thinner pore wall and are thermally less stable (<873 K). Preparation in acidic conditions leads to materials with unique structures and properties. Ti incorporation in those materials is a challenging task. Several micro-mesoporous and amorphous materials have been known but all them have the above disadvantages. A material with, ordered, three-dimensional pore structure and Ti in framework locations would enhance the reaction rate by facilitating diffuse of reactant and product molecules through the interconnected, ordered, 3-dimensional pores which is otherwise not possible with 2-dimensional titanosilicate structures. Because of the facile diffusion of reactant and product molecules in 3-dimensional mesopore structures, pore plugging/blocking which is normally anticipated and observed leading to deactivation of microporous titanosilicate materials can be avoided. In view of all these advantageous features and their applicability in transformation of bulky molecules of pharmaceutical interest, a stable, ordered mesoporous, titanosilicate with three-dimensional pore structure and Ti located in the framework tetrahedral Si location is more efficient and hence, desirable for oxidation and acid catalyzed aminolysis of epoxides reactions. Often in the synthesis of titanosilicates, instead of titanium getting substituted in the framework it gets precipitated as a separate nano phase on the mesoporous surfaces. Preparation of three dimensional mesoprous materials with thick pore walls and titanium being isolated and substituted in the framework tetrahedral location is a challenging task.
Anuj Kumar et al (Chem. Commun. pages 6484-6486, year 2009) reported the preparation of Ti—SBA-12 and its application in oxidation reactions. This material belongs to the class of hexagonal pore arrangement. While Ti possesses tetrahedral structure, it has a tripodal Ti(OSi)3OH geometry as revealed by EPR and UV-visible spectroscopy and selectivity for epoxide in cyclohexene conversion is not 100%. Also the conversion of cyclooctene a bulkier molecule is only 61%. This material has low wall thickness. Shen et al (J. Mater. Sci., vol. 42, pp. 7057-7061, year 2007) reported Ti—SBA-16 but Ti in this material has octahedral geometry (UV band 330 nm; Raman band 144 cm−1) and present mostly as an extra framework anatase-like titania phase. Ti is not substituted in the framework and hence, will not be active in oxidation reactions. Further oxidation reactions catalyzed by this Ti—SBA-16 will lead to non-selective oxidations and decomposition of oxidants. Also, this material has lower surface area and pore diameter leading to lower activity. Carlos et al (Catal. Today Vol. 107-108, pp. 578-588, year 2005) reported preparation of Ti—SBA-16 by post synthesis method. Again Ti is present as a extra-lattice anatase phase. Ti is located on the surface of pores but not in the framework as an active form for oxidation reactions. Shah et al in “Direct synthesis of Ti-containing SBA-16-type mesoporous material by the evaporation-induced self-assembly method and its catalytic performance for oxidative desulfurization” published in Journal of Colloid and Interface Science; Volume 336, Issue 2, 15 Aug. 2009, Pages 707-711 describes novel Ti-containing SBA-16-type mesoporous material (with various Ti loadings of 5, 10, and 15 wt %), synthesized by an evaporation-induced self-assembly method using F127 copolymer as template. But this material leads to a different disordered material affecting diffusion and reaction rates adversely.
An article titled “Direct Synthesis of Titanium Incorporated SBA-16 Molecular Sieves” by Govindasamy Chandrasekar et. al; published in Theories and Applications of Chem. Eng., 2008, Vol. 14, No. 1 describes highly ordered three dimensional (3-D) cubic TiSBA-16 molecular sieves with different nSi/nTi ratio prepared through direct synthesis under highly acidic condition. The structure and the textural properties of the materials were characterized by X-ray diffraction, N2 physisorption, SEM, and TEM analysis. The nature and the coordination of the Ti atoms in SBA-16 prepared with various Ti content were investigated by UV-vis spectroscopy. Further, states that the Ti atoms are well-dispersed and mostly occupy the tetrahedral coordination.